Dynamin, a crucial factor in endocytosis, is a member of a family of GTPases that participates in membrane fission. It was initially proposed to act as a machine that constricts and cuts the neck of nascent vesicles in a GTP-hydrolysis-dependent reaction, but subsequent studies suggested alternative models. Here we monitored the effect of nucleotides on dynamin-coated lipid tubules in real time. Addition of GTP, but not of GDP or GTP-gammaS, resulted in twisting of the tubules and supercoiling, suggesting a rotatory movement of the helix turns relative to each other during GTP hydrolysis. Rotation was confirmed by the movement of beads attached to the tubules. Twisting activity produced a longitudinal tension that was released by tubule breakage when both ends of the tubule were anchored. Fission also occurred when dynamin and GTP were added to lipid tubules that had been generated from liposomes by the motor activity of kinesin on microtubules. No fission events were observed in the absence of longitudinal tension. These findings demonstrate a mechanoenzyme activity of dynamin in endocytosis, but also imply that constriction is not sufficient for fission. At the short necks of endocytic vesicles, other factors leading to tension may cooperate with the constricting activity of dynamin to induce fission.

SUMMARY During spermatogenesis, germ cells initially expand exponentially through mitoses. A majority of these cells are then eliminated through p53-mediated apoptosis to maintain germline homeostasis [1–4]. However, the activity of p53 must be precisely modulated, especially suppressed in postmitotic spermatogenic cells, to guarantee robustness of spermatogenesis. Currently, how the suppression is achieved is not understood. Here, we show that Pumilio 1, a posttranscriptional regulator, binds to mRNAs representing 1527 genes, with significant enrichment for mRNAs involved in pathways regulating p53, cell cycle, and MAPK signaling. Particularly, eight mRNAs encoding activators of p53 are repressed by Pumilio 1. Deleting Pumilio 1 results in strong activation of p53 and apoptosis mostly in spermatocytes, which disrupts sperm production and fertility. Removing p53 reduces apoptosis and rescues testicular hypotrophy in Pumilio 1-null mice. These results indicate that key components of the p53 pathway are coordinately regulated by Pumilio 1 at the posttranscriptional level, which may exemplify an RNA operon.

We previously demonstrated that the Kaposi's sarcoma-associated herpesvirus polyadenylated nuclear RNA contains a 79-nt cis-acting element, the ENE, which allows intronless polyadenylated transcripts to accumulate to high nuclear levels by protecting them from rapid degradation. We proposed a model based on the predicted structure of the ENE in which a U-rich internal loop hybridizes with the 3-polyadenylate (polyA) tail to sequester it from exonucleolytic attack. We have tested this model by mutational analysis of the ENE. Point mutations in the predicted U-rich internal loop and in the flanking stems abolish the ENE's ability to (i) interact with the polyA tail, (ii) inhibit deadenylation in vitro, and (iii) stabilize transcripts in vivo. In all but one case, compensatory mutations in the flanking stems restore ENE activities, demonstrating the importance of these stems and uncovering a unique role for the loop-proximal G-C base pair in the lower stem. Increasing the U content of the U-rich internal loop surprisingly decreases stability in vivo but does not affect deadenylation in vitro, comparable to the effects of deleting certain ''unstructured'' regions of the ENE. Taken together, our data support the formation of the proposed ENE secondary structure in vivo and argue that the specific ENE structure inhibits rapid RNA decay in cis by engaging in a limited set of base-pairing interactions with the polyA tail.deadenylation ͉ polyadenylated nuclear RNA ͉ polyadenylation ͉ RNA degradation ͉ RNA structure R NA decay rates control the steady-state levels of an mRNA transcript, so an understanding of the determinants of RNA stability is essential to understanding gene expression. Specific cis-acting RNA elements can increase stability or accelerate decay (1, 2). A well studied example of regulated decay occurs with transcripts that contain an AU-rich element (ARE), which normally confers a short half-life but can stabilize the mRNA in response to environmental cues. RNA decay pathways also serve ''quality control'' functions (3), selectively degrading aberrant transcripts to ensure the fidelity of gene expression. Examples include nonsense-mediated, nonstop, and no-go decay pathways, whereby mRNAs are rapidly and selectively degraded when the translating ribosome encounters a premature termination codon, no termination codon or becomes translationally stalled, respectively (4-8). Surveillance mechanisms are not unique to mRNAs; noncoding and intergenic transcripts are also subject to quality control pathways (9-15).In many cases, the first and rate-limiting step of mRNA decay is removal of the 3Ј polyadenylate (polyA) tail (1, 2). In both yeast and mammalian cells, mRNA deadenylation generally precedes decapping and subsequent 5Ј to 3Ј and 3Ј to 5Ј exonucleolytic decay. Indeed, substrates undergoing both AREmediated decay and nonsense-mediated decay show increased deadenylation rates in vivo (16-18). Moreover, mRNAs targeted for destabilization by microRNAs have increased deadenylation rates (19,20).The Kaposi's ...

SUMMARY Glaucoma is the leading cause of irreversible blindness and is characterized by the death of retinal ganglion cells (RGCs). Recent studies have implicated pro-inflammatory microglia, macrophages, and A1 astrocytes in the pathogenesis of neurodegenerative diseases. The role of pro-inflammatory, neurotoxic A1 astrocytes in glaucoma is just beginning to be explored. Using a mouse model of glaucoma, we demonstrate that ocular hypertension is sufficient to trigger production of C1q, interleukin-1α (IL-1α), and tumor necrosis factor α (TNF-α), three cytokines necessary and sufficient to drive the formation of A1 astrocytes. Upregulation of these cytokines occurs first in CD11b + CD11c + cells followed by CD11b + CD11c − cells. Ablation of this pathway, by either genetic deletions of C1qa, IL-1α, and TNF-α, or treatment with glucagon-like peptide-1 receptor agonist NLY01, reduces A1 astrocyte transformation and RGC death. Together, these results highlight a neuroinflammatory mechanism of glaucomatous neurodegeneration that can be therapeutically targeted by NLY01 administration.